1. Field of the Invention
The present invention relates to ophthalmic instruments for examining and testing eyes.
2. Description of the Prior Art
For carrying out examination or photography of an eye it is very important to preliminarily position the appropriate instrument correctly relative to the subject eye. For example, in setting a retinal camera there are required two kinds of adjustments. Initially, the camera must be adjusted vertically (up-and-down) and horizontally (left-and-right), while observing the subject eye from the side of the camera, to a position at which the illumination light emerging from the objective lens is properly incident upon the pupil of the subject eye. This kind of adjustment is usually called "alignment". Secondly, while viewing an image of the fundus of the subject eye through the finder, the camera is adjusted by moving it forwards or backward to a position at which the image of the fundus becomes clearly visible. In the art, this kind of adjustment is generally called "adjustment of working distance".
For ophthalmoscopy there are known such type of instruments in which infrared light is used in examining the subject eye or in illuminating the fundus for focusing. The iris of the eye is not sensitive to infrared light and therefore no constriction of the pupil is caused by illumination with infrared light. Accordingly, the use of infrared light has an advantage in that it permits examining, measuring, testing or photographing of the eye in a state of spontaneous dilatation.
However, the illumination light of infrared rays is not visible. Therefore, the adjustment of such type of apparatus must be made by relying upon only the examiner's sense or skillfulness. Also, the image formed with infrared light must be converted into a visible image for observation by using an infrared converter because no infrared image is visible to the naked eye. By this conversion, however the quality of the image is reduced to the extent that the visible image is not longer useful for the adjustment of working distance. If a photograph of the fundus is taken under the condition of insufficiently adjusted working distance, then there will be produced a defective image which is damaged by a portion of fundus illuminating light reflected upon the cornea and then introduced into the film. In measuring the eye, such insufficient adjustment of working distance will cause measuring error to be increased.
U.S. Pat. No. 3,871,772 has disclosed an eye examining instrument which enables one to carry out not only alignment but also working distance adjustment. But, the finder belonging to the instrument can not be used to effect observation during adjustment. The apparatus disclosed therein is provided with a separate finder as the observation finder for adjustment.